Gas at high pressure is often stored, transported or delivered to a usage point in a cylinder. Cylinders generally have a cylindrical shape, i.e. a generally circular cross-section with one long axis perpendicular to the circular cross-section. Cylinders are often positioned with the long axis vertically oriented when they are used to store, transport or deliver gas.
A problem which arises when gas mixtures are used with cylinders is the stratification of individual components of the gas mixture, especially when the individual components have significantly differing densities. A cylinder is generally filled first with one component of the gas mixture and then with the second component. If the gas mixture has three or more components, each subsequent component is sequentially charged into the cylinder. The charging is generally carried out through a valve located at the neck or narrowed portion of the cylinder. The sequential charging of each gas mixture component to the cylinder contributes to an initial stratification of the individual gas mixture components. Furthermore, if the cylinder remains in a single position for a length of time, the stratification proceeds further as gas components of differing densities preferentially rise or fall within the cylinder.
The stratification is generally not complete; there is some of each component in all parts of the cylinder. The stratification, rather, results in the cylinder having a gas mixture richer in the lighter component or components in the upper portion of the cylinder than what the gas mixture specification calls for, and a gas mixture richer in the heavier component or components in the lower portion of the cylinder than what the gas mixture specification calls for. This stratification may cause significant quality and other problems because it causes a gas mixture to be delivered from the cylinder which deviates from the gas mixture specifications. This is a particular problem in certain applications such as in welding where even a small deviation from the shielding gas specification may have a significant impact on weld quality.
One method which has been used to address this gas mixture stratification problem is to roll or shake each cylinder prior to use. This procedure has met with limited success because it is time consuming and cumbersome, and moreover may be dangerous because of the cylinder weight and the high pressures at which the gas mixture is stored.
Furthermore if gas were to be injected into the cylinder through a tube from the valve, the tube would remain filled with the last to be charged component of the gas mixture. Thus, upon delivery of the gas mixture from the cylinder to a usage point, the first portion of gas delivered would not be the gas mixture but, rather, would be essentially pure gas component which remained in the tube. This problem could be addressed by bleeding off the first portion of the cylinder contents until this pure component in the tube is fully withdrawn. However, such bleeding off would result in some wastage.
A recent significant advance in the field of gas cylinders has been the cylinder described and claimed in U.S. Pat. No. 4,461,657--Rana et al and U.S. Pat. No. 4,578,113--Rana et al. This new cylinder enables the use of much higher pressures for the gas within the cylinder. Thus significantly more gas can be put into a cylinder of a given size than was heretofore possible with conventional cylinders. This improves the efficiency of the gas transport and delivery operation.
However, in the case of gas mixtures, the new high pressure cylinder would exacerbate the problem of pure gas component in a tube because the tube would be filled with pure component at much high pressure, thus taking longer to clear the tube of pure component and begin delivery of the specified gas mixture. Accordingly, wastage would be increased serving to negate some of the advantage attainable with the high pressure cylinder.
Accordingly, it is an object of this invention to provide a cylinder for the containment of gas, and specifically gas mixtures, which will enable more efficient operation than is possible with heretofore available cylinders.
It is another object of this invention to provide a method for charging a gas cylinder which will enable more efficient operation than is possible with heretofore available gas cylinder charging methods.
It is a further object of this invention to provide a method for delivering gas from a gas cylinder which will enable more efficient operation than is possible with heretofore available gas delivery methods.